The locus of AGNs in the evolutionary path from star-forming
galaxies to quiescent galaxies may provide valuable information
about the role of AGNs. Previous studies have revealed some
interesting trends. For example, some studies found that the host galaxies of
AGNs appear to be located at the green valley, which is the
transition region from star-forming to quiescent galaxies
[1]}, [2]}, [3]}. Moreover, studies based on AGN
clustering, weak lensing and galaxy groups all suggests that optically
selected AGNs at low redshift reside preferentially in halos of roughly
\(M_{\rm h,A}=10^{12}\, h^{-1}{\rm M_\odot }\)  [4]}, [5]}, [6]}, [7]},
similar to the pivot halo mass, suggesting
that optical AGNs may be at a special stage of galaxy evolution.
It is thus interesting to understand why optical AGNs
favor halos of \(M_{\rm h,A}=10^{12}\, h^{-1}{\rm M_\odot }\)  and what processes,
in these halos are responsible for triggering AGN
activities.
